This disclosure relates to polycarbonates derived from aliphatic compounds, and in particular to polycarbonates derived from an aliphatic diol, methods of manufacture thereof, and articles comprising the polycarbonates.
Polycarbonate homopolymers and copolymers derived from aliphatic diols, particularly isosorbides (i.e., 2,6-dioxabicyclo[3.3.0]octane-4,8-diol and isomers thereof), are of great interest to the chemical industry because aliphatic diols can be produced from renewable resources, namely sugars, rather than from petroleum feed stocks. However, there are particular challenges associated with producing polycarbonates from aliphatic diols such as isosorbide.
The primary commercial method for preparing polycarbonates is by interfacial polymerization in a methylene chloride/water mixture using phosgene and alkali. Interfacial processes for producing isosorbide homopolycarbonate in pyridine-containing solvent mixtures at low temperatures have been described by Kricheldorf et al. in Macromolecules vol. 29, p. 8077 (1996). Kricheldorf et al. describe the preparation of isosorbide homopolycarbonate by converting isosorbide to the bischloroformate, followed by interfacial polymerization. The polymer obtained exhibited a Tg of 144 to 155° C. However, pyridine is not a suitable solvent for large-scale processes. In addition, U.S. Pat. No. 4,506,066 discloses attempts to prepare copolycarbonates derived from isosorbide and bisphenol A by interfacial polymerization in an alkaline water/methylene chloride mixture with phosgene. Only bisphenol A polycarbonate was obtained and no incorporation of isosorbide was observed. It is theorized that interfacial methods are not commercially suitable for preparing homopolycarbonate derived from aliphatic diols, such as isosorbide, because the higher diol solubility in water impedes interphase transfer and the acidity of the diol protons is too low for the polymerization to proceed at an adequate rate in pH ranges suitable for commercial-scale interfacial phosgenation.
Another method of synthesizing polycarbonates is by melt polymerization, for example by melt transesterification of a dihydroxy compound with a source of carbonate units, such as diphenyl carbonate (DPC), in the presence of a catalyst and the absence of solvent. GB 1,079,686 discloses preparation of isosorbide homopolycarbonate by melt transesterification with DPC in the absence of a catalyst. DE 3,002,276 describes the preparation of lower molecular weight oligomers of copolycarbonates by the reaction of isosorbide with bisphenol A, 4,4′-dihydroxydiphenyl sulfide, or 4,4′-dihydroxy biphenyl and DPC using a disodium salt of bisphenol A as a transesterification catalyst. The phenyl carbonate terminal groups of the oligomers were then hydrolyzed and interfacially polymerized to produce high molecular weight copolycarbonates. U.S. Pat. No. 4,506,066 describes the melt polymerization of isosorbide with DPC at 220° C. to produce a pale brown polymer along with insoluble constituents. In this study it was presumed that during melt polymerization branching had occurred, leading to the formation of an insoluble inhomogeneous product. Thus, it was concluded that one-step melt polymerization processes are not suitable for the preparation of isosorbide homo- and copolycarbonates. A detailed polymerization study carried out by Kricheldorf et al. supports this conclusion, as one-step polymerization of isosorbide diphenyl carbonate with various diphenols catalyzed by ZnO was reported to only lead to the formation of products that were insoluble in all common solvents tested. Macromolecules, vol. 29, p. 8077 (1993).
U.S. Pat. No. 7,138,479 discloses an activated carbonate melt process to synthesize an isosorbide copolycarbonate having a random arrangement of structural units. Isosorbide homopolycarbonates having molecular weights (gel permeation chromatography, polystyrene standards) of 16,060 g/mol and 20,678 g/mol were obtained using non-activated and activated melt polymerization processes, respectively. However, such molecular weights are not sufficiently high for use in many, if not all, commercial applications.
Accordingly, there remains a need in the art for methods for the manufacture of high molecular weight homopolycarbonates and copolycarbonates derived from aliphatic diols, specifically isosorbide.